An anesthetic, or combination of anesthetics, may be delivered to a patient in order to produce the effects of sedation, analgesia, and neuro-muscular block, broadly referred to as anesthesia. Different anesthetics produce different effects and degrees of effects, and therefore, must be carefully delivered to the patient. When the anesthetic, or combination of anesthetics, is delivered in a gaseous form for patient inhalation, the anesthetic is combined with one or a combination of carrier gases for delivery to the patient. A vaporizer combines these two or more gases before delivery to the patient.
Acoustic wave time-of-flight (time-of-flight) t is the ratio of distance traveled (distance) D by an acoustic wave to acoustic wave speed (speed) v. As seen in FIG. 1, speed v has two components, one due to acoustic wave speed in the media (speed in media) vs in which it travels, and one due to the speed of the media itself (media speed) vm, arising from flow of the media (flow) F. Speed in media vm is related to media heat capacity ratio (heat capacity ratio) γ, media temperature (temperature) T, and media molar mass (molar mass) M. Direct measurement of time-of-flight t, rather than speed v, is possible using time-of-flight sensors (sensor) 5. Practitioners will recognize that a sensor can be a fully integrated device located at the point of measurement, comprised of a transducer and other components required to acquire and report a measurement, or that a sensor can be a distributed device, minimally having a transducer located at the point of measurement, with other components located elsewhere. The inclination angle α of the sensor mounting versus the direction of flow (inclination) determines the influence of flow F on speed v. These relationships are well known to practitioners and are summarized below.
                    t        =                  D          v                                    (        1        )                                v        =                              v            s                    ±                                    v              m                        ⁢            sin            ⁢                                                  ⁢            α                                              (        2        )                                                      v            s                    =                                                    γ                ⁢                                                                  ⁢                RT                            M                                      ⁢                                  ⁢                  t          ⁢                                          ⁢          is          ⁢                                          ⁢          time          ⁢                      -                    ⁢          of          ⁢                      -                    ⁢          flight                ⁢                                  ⁢                  D          ⁢                                          ⁢          is          ⁢                                          ⁢          distance                ⁢                                  ⁢                  v          ⁢                                          ⁢          is          ⁢                                          ⁢          speed                ⁢                                  ⁢                              v            s                    ⁢                                          ⁢          is          ⁢                                          ⁢          speed          ⁢                                          ⁢          in          ⁢                                          ⁢          media                ⁢                                  ⁢                              v            m                    ⁢                                          ⁢          is          ⁢                                          ⁢          media          ⁢                                          ⁢          speed                ⁢                                  ⁢                  α          ⁢                                          ⁢          is          ⁢                                          ⁢          inclination                ⁢                                  ⁢                  γ          ⁢                                          ⁢          is          ⁢                                          ⁢          heat          ⁢                                          ⁢          capacity          ⁢                                          ⁢          ratio                ⁢                                  ⁢                  R          ⁢                                                            ⁢                                                          ⁢          is          ⁢                                          ⁢          molar          ⁢                                          ⁢          gas          ⁢                                          ⁢          constant                ⁢                                  ⁢                  T          ⁢                                          ⁢          is          ⁢                                          ⁢          temperature                ⁢                                  ⁢                  M          ⁢                                          ⁢          is          ⁢                                          ⁢          molar          ⁢                                          ⁢          mass                                    (        3        )            
Gas composition affects speed v by modifying the speed in media vm. This occurs as gas composition affects both heat capacity ratio γ and molar mass M, dependent on the amounts of the component gases present based on volumetric concentration (concentration) c. Flow F affects speed v by modifying the media speed vm. These relationships are also well known to practitioners and are summarized below.
                    γ        =                  1          +                                    (                                                c                                                            γ                      1                                        -                    1                                                  +                                                      1                    -                    c                                                                              γ                      2                                        -                    1                                                              )                                      -              1                                                          (        4        )                                M        =                              cM            1                    +                                    (                              1                -                c                            )                        ⁢                          M              2                                                          (        5        )                                                      v            m                    =                      F            A                          ⁢                                  ⁢                  c          ⁢                                          ⁢          is          ⁢                                          ⁢          concentration          ⁢                                          ⁢          of          ⁢                                          ⁢          component          ⁢                                          ⁢          1                ⁢                                  ⁢                              γ            1                    ⁢                                          ⁢          is          ⁢                                          ⁢          heat          ⁢                                          ⁢          capacity          ⁢                                          ⁢          ratio          ⁢                                          ⁢          of          ⁢                                          ⁢          component          ⁢                                          ⁢          1                ⁢                                  ⁢                              γ            2                    ⁢                                          ⁢          is          ⁢                                          ⁢          heat          ⁢                                          ⁢          capacity          ⁢                                          ⁢          ratio          ⁢                                          ⁢          of          ⁢                                          ⁢          component          ⁢                                          ⁢          2                ⁢                                  ⁢                              M            1                    ⁢                                          ⁢          is          ⁢                                          ⁢          molar          ⁢                                          ⁢          mass          ⁢                                          ⁢          of          ⁢                                          ⁢          component          ⁢                                          ⁢          1                ⁢                                  ⁢                              M            2                    ⁢                                          ⁢          is          ⁢                                          ⁢          molar          ⁢                                          ⁢          mass          ⁢                                          ⁢          of          ⁢                                          ⁢          component          ⁢                                          ⁢          2                ⁢                                  ⁢                  F          ⁢                                          ⁢          is          ⁢                                          ⁢          volumetric          ⁢                                          ⁢          flow                ⁢                                  ⁢                  A          ⁢                                          ⁢          is          ⁢                                          ⁢          flow          ⁢                                          ⁢          cross          ⁢                      -                    ⁢          sectional          ⁢                                          ⁢          area                                    (        6        )            
In general, application of the previous relationships to concentration c requires removing the effect of flow F using time-of-flight measurements t of acoustic waves traveling upstream and downstream versus flow F, then eliminating the media speed vm term using equations (1) and (2). In general, application of the previous relationships to flow F requires removing the effect of speed in media vs using time-of-flight measurements t of acoustic waves traveling upstream and downstream versus flow F, then eliminating the speed in media vs term using equations (1) and (2). These techniques are well known to practitioners and the equations involved are shown below.
                              t          u                =                  D                      v            u                                              (        7        )                                          v          u                =                              v            s                    -                                    v              m                        ⁢            sin            ⁢                                                  ⁢            α                                              (        8        )                                          t          d                =                  D                      v            d                                              (        9        )                                                      v            d                    =                                    v              s                        +                                          v                m                            ⁢              sin              ⁢                                                          ⁢              α                                      ⁢                                  ⁢                              t            u                    ⁢                                          ⁢          is          ⁢                                          ⁢          upstream          ⁢                                          ⁢          time          ⁢                      -                    ⁢          of          ⁢                      -                    ⁢          flight                ⁢                                  ⁢                              v            u                    ⁢                                          ⁢          is          ⁢                                          ⁢          upstream          ⁢                                          ⁢          speed                ⁢                                  ⁢                              t            d                    ⁢                                          ⁢          is          ⁢                                          ⁢          downstream          ⁢                                          ⁢          time          ⁢                      -                    ⁢          of          ⁢                      -                    ⁢          flight                ⁢                                  ⁢                              v            d                    ⁢                                          ⁢          is          ⁢                                          ⁢          downstream          ⁢                                          ⁢          speed                                    (        10        )            
Taking anesthetic to be a first component and carrier gas to be a second component, combining equations (3)-(10) implies the general relationship below exists among the parameters at the inlet and outlet of a vaporizer. Inlet gas consists only of carrier gas. Outlet gas consists of a combination of anesthetic and carrier gas. Practitioners will recognize that the concentration c of anesthetic at the outlet of the vaporizer is synonymous with vaporizer output (output) and is the fundamental vaporizer parameter to be controlled.c=ƒ(tiu,tid,tou,tod,γcg,γa,Ti,To,Mcg,Ma)  (11)                ƒ( . . . ) denotes “is a function of”, with exact form dependent on context        c is output        tiu is inlet upstream time-of-flight        tid is inlet downstream time-of-flight        tou is outlet upstream time-of-flight        tod is outlet downstream time-of-flight        γcg is carrier gas heat capacity ratio        γa is anesthetic heat capacity ratio        Ti is inlet temperature        To is outlet temperature        Mcg is carrier gas molar mass        Ma is anesthetic molar mass        
Referring back to earlier discussion, combining equations (6)-(10) and converting volumetric flow to volumetric flow at standard conditions using the Ideal Gas Law, implies the general relationships below exist.Fi=ƒ(tiu,tid,Pi,P,Ti,T)  (12)Fo=ƒ(tou,tod,Po,P,To,T)  (13)                Fi is inlet flow at standard conditions        Pi is inlet pressure        P is standard pressure        Ti is inlet temperature        T is standard temperature        Fo is outlet flow at standard conditions        Po is outlet pressure        To is outlet temperature        
U.S. patent application Ser. No. 12/648,602 describes a scheme (prior vaporizer) to control output c from a vaporizer using a negative feedback controller based on time-of-flight measurements ti, to at the inlet and outlet of the vaporizer as suggested by equation (11). This involves computing a target outlet time-of-flight {circumflex over (t)}o from the inlet time-of-flight ti, the anesthetic identity ID, and the commanded output co. Then, an error signal e is formed for the outlet time-of-flight to. Finally, an actuator throttling the amount of anesthetic delivered from the vaporizer is driven from the error signal e to control output. Judicious mounting of sensors at the vaporizer inlet and outlet to negate the effect of flow F on speed v, maintaining the inlet and outlet at similar temperatures T to negate the effect of temperature variation, and the use of singular nominal values for heat capacity ratio γ and molar mass M for all possible combinations of carrier gas to make the vaporizer independent of the carrier gas source enable a simpler, less costly, and more available vaporizer than suggested by equation (11). The prior vaporizer was reported applicable to controlling a wide range of anesthetics, including, but not limited to, Desflurane, Enflurane, Halothane, Isoflurane, Sevoflurane, and Xenon in a wide range of carrier gases, including, but not limited to, air, carbon dioxide, heliox, nitrous oxide, and oxygen.
Risk management regulations require control measures be provided in a vaporizer to make it safe for patient use, with an inherently safe design being optimum. An inherently safe vaporizer design operates in such a way that it self detects unsafe conditions and shuts down anesthetic output without operator setup, monitoring, or intervention. The primary patient hazards associated with vaporizer use are over and under delivery of anesthetic. It is an objective of the present application to create a method whereby the prior vaporizer is extended to be an inherently safe design with respect to over and under delivery of anesthetic. A secondary patient hazard associated with vaporizer use is loss of carrier gas. It is an objective of the present application to create a method whereby the prior vaporizer is extended to monitor carrier gas loss within the vaporizer.
For a vaporizer operator (operator), two beneficial quantities to know are (1) anesthetic consumption (consumption) during a time period for patient billing purposes, and (2) anesthetic delivery time remaining (time remaining) at the current output and carrier gas flow input for vaporizer refilling purposes. The present application includes a method whereby the prior vaporizer is extended to monitor consumption. The present application also includes a method whereby the prior vaporizer is extended to monitor time remaining.
Application of time-of-flight t to flow F requires an implementation where flow F affects speed v. In the prior art, this is typically accomplished using a physical configuration where the sensors 5 are positioned with an inclination α to the direction of flow F, as seen in FIG. 1. In this manner, media speed vm has a component along the direction of acoustic wave 7 travel. Flow meter design, however, is a tradeoff of several design parameters that affect implementation feasibility, performance, and cost.
Three significant design parameters of time-of-flight flow meters used for gas composition and flow are (1) time-of-flight magnitude, (2) time-of-flight change magnitude, and (3) flow meter volume. The former two parameters must be reconciled with commercially available sensors and electronics, with maximization of both time-of-flight magnitude and time-of-flight change magnitude being preferred. The latter parameter of the three determines the speed of flow meter response to composition changes, as media in the flow meter must be fully exchanged for proper detection, with minimization of flow meter volume being preferred. The present application includes a new flow meter configuration whereby the tradeoff between the time-of-flight change magnitude and flow meter volume is improved versus the prior art.